1. Field of the Invention
The present invention relates to a silicon gyroscope for detecting the rotatory angular velocity based on the Coriolis force which arises when the vibrator of the gyroscope turns while vibrating, and also relates to a method of driving the silicon gyroscope. Particularly, the invention relates to a silicon gyroscope which is capable of stably detecting the angular velocity and a silicon gyroscope which has enhanced sensitivity without the need of supply of a high drive voltage, and also relates to a method of driving the silicon gyroscopes.
2. Description of the Prior Art
There has been developed recently a compact vibratory gyroscope for use in a navigation system equipped on motor vehicles, an attitude controller of unmanned vehicles, and a field swing preventive device of video cameras. This vibratory gyroscope consists of a vibrator having three parallel elastic arms separated by two notches, a drive means for vibrating the elastic arms, and a means of detecting the vibration component which is orthogonal to the direction of vibration of the elastic arms resulting from the turning of the vibrator. The vibrator of the gyroscope is made of piezoelectric ceramics or constant elasticity metal (elinvar).
The vibrator of piezoelectric ceramics necessitates only the formation of electrodes for current conduction by basing the operation on its own piezoelectricity, and it can be simple in structure. However, piezoelectric ceramics generally has a small Q value of vibration (ranging from 20 to 1000). Therefore, it cannot be expected to have a large resonance-based displacement amplification effect. It also has drawbacks of the need of large input energy and the heating caused by the large input energy.
The piezoelectric ceramics material also has a large temperature-dependent variation of Young's modulus (ranging from 10 to 200 ppm) and a large linear expansion coefficient (ranging from 10 to 50 ppm). On this account, the vibrator of piezoelectric ceramics varies in its dimensions in response to the variation of environmental temperature, resulting possibly in a sensor output variation. It also suffers from the temperature dependence of piezoelectricity (ranging from 100 to 5000 ppm) and thus can have output fluctuation in response to the variation of environmental temperature even in the absence of variation of dimensions.
Moreover, the piezoelectric sensor heats up by itself, and therefore it is liable to have output fluctuation during the period after it is turned on until it reaches the steady state temperature even at a constant environmental temperature. For example, for a gyroscope sensor which bases the azimuth detection on the integration of angular velocity output, a fluctuation of null value of the output results in an error of detected azimuth.
In the case of a vibrator formed of constant elasticity metal, piezoelectric elements are glued to the elastic arms and energized so that the arms vibrate, causing the whole vibrator of constant elasticity metal to vibrate. This vibrator is more complex in structure than the vibrator of piezoelectric ceramics mentioned previously.
FIG. 35 is a perspective view of a vibrator of a conventional gyroscope made from constant elasticity metal, and FIG. 36 is a cross-sectional view of the vibrator arm. The vibrator 200 has elastic arms 201a,201b and 201c which extend in parallel to each other, on which are glued piezoelectric elements 202a-202l, with electrodes 203a-203l for current conduction being formed thereon.
The vibrator of constant elasticity metal, with the piezoelectric elements 202 being glued thereon, has problems similar to the above-mentioned problems inherent to piezoelectric ceramics, and further has a problem of the divergence of resonant frequencies of drive and detection caused by the distortion of vibrator 200 due to different thermal expansion coefficients of different materials glued together, a problem of output fluctuation caused by the variation of vibration amplitude of the vibrator 200, a problem of the influence of vibration of the vibrator 200 when there is a gap between the piezoelectric element 202 and the vibrator 200, and a problem of the influence on the output signal attributable to the Coriolis force.
In regard to the conventional drive method for the. silicon gyroscope, a voltage is applied between the vibrator and the driving electrodes in the vibrator drive direction so that the vibrator is driven by the electrostatic force, and the value of displacement of the vibrator caused by the Coriolis force is detected in terms of the variation of static capacitance between the vibrator and the detecting electrodes.
However, this conventional drive method not only needs to deal with a small static capacitance (0.1 to 3 pF) in a quiescent state, but also an extremely small variation of static capacitance (5 to 500 aF) arising in response to a displacement of vibrator caused by the Coriolis force. In addition, a C-V conversion circuit which converts the static capacitance into a voltage value is extremely susceptible to external noises attributable to electromagnetic induction or the like due to a high input impedance of the circuit. Moreover, due to the device structure in which the driving electrodes and detecting electrodes are located closely on the vibrator, it is difficult to prevent the induction noise created by the driving electrodes from leaking to the high-impedance detecting electrodes.
The static capacitance varies in response to the Coriolis force at the same frequency as driving of the vibrator, making it difficult to separate the signal from noise and thus causing the C-V conversion circuit to have a smaller gain, resulting in a smaller sensitivity of detection of angular velocity.
The conventional drive method bases the flexure of vibrator on the expansion and contraction of piezoelectric element, and therefore the vibrator is liable to twist in the motion of drive vibration due to the unevenness of piezoelectric material. Similarly, at detection, the displacement of vibrator created by the Coriolis force in the direction orthogonal to the direction of drive vibration fluctuates. Error of orthogonality between the vibration direction of drive and the vibration direction of detection creates a mechanical coupling of the driving signal with the vibration of detection, and affects the output signal, resulting in a drift or offset of the output signal.
From the foregoing viewpoints, the conventional gyroscope and the associated drive method are deficient seriously in implementing the stable detection of angular velocity.